Microwave modulator using single crystal ferrite



Feb.w15, 1966 J. CARVELAS ETAL 3,235,819

MICROWAVE MODULATOR USING SINGLE CRYSTAL FERRITE Filed April 2, 1962 2 Sheets-Sheet 1 FIG. 1

g 1; 34 Z is z DC r3 GEN FIG. 2 FIG. 3 2| IQ MAX z 25 23 E 8500 INVENTO.

JAMES CARVEL R M. VESPER WEICHARDT ICHARD BY HEINZ H ATTORNEY.

FREQUENCY, MCPS Feb. 15, 1966 MICROWAVE MODULATOR USING SINGLE CRYSTAL FERRITE Filed April 2, 1962 J. CARVELAS ETAL 3,235,819

2 Sheets-Sheet 2 MAX -f TRANSMISSION s19? aa'oo 8803 FREQUENCY, MCPS FIG. 4

COM -69 56 CO/L INVENTOR. JAMES ARVELAS RICHA M. VESPER BY HEINZ H. WEICHARDT ATTORNEY.

United States Patent Ofiice 3,235,819 Patented Feb. 15, 1966 3,235 819 MICROWAVE MODULATOR USING SINGLE CRYSTAL FERRITE James Carvelas, Hawthorne, Richard M. Vesper, Bronx, and Heinz H. Weichardt, Spring Valley, NY. assignors to General Precision, 1116., a corporation of Delaware Filed Apr. 2, 1962, Ser. No. 184,106 6 Claims. (Cl. 332-52) This invention relates to microwave modulators and switches which employ a component composed of a magnetic material exhibiting magnetic resonance.

One class of magnetic material includes such substances as yttrium-iron garnet which are capable of ferromagnetic resonance. It has been discovered that the frequency of ferromagnetic resonance of such a substance is changed when mechanical force is applied to the substance. The present invention utilizes this property.

It is obvious that, when mechanical force is applied to a single crystal of yttrium-iron garnet, a Strain is set up in the crystal lattice. The exact mechanism by which this strain affects the ferromagnetic resonance frequency is not known, but the fact that the frequency is affected has been demonstrated by test.

In order to employ this property effectively, it seems to be necessary that the substance be in the form of a single crystal. It is also considered necessary to confine the material used to those materials having a narrow ferromagnetic resonance bandwidth of preferably not more than 15 gauss.

Examples of substances having properties making them useful in this invention include plain yttrium-iron garnet, doped yttrium-iron garnet, and some other garnets. In general, any ferrite having a sufliciently narrow resonance line width is suitable for use in this invention.

The substance employed in this invention, such as single-crystal yttrium-iron garnet, is preferably but not necessarily employed in the form of a sphere. Alternatively it will operate successfully in the shape of a disc and other shapes. The surface should be made as smooth as possible to reduce the resonance line width.

It is expedient to couple the resonant substance to a microwave field by placing the substance in a microwave cavity at a point of high radio frequency magnetic field strength. The use of a resonant cavity is, however, not essential, and any other method of coupling a variable impedance device to a microwave field may be employed.

As an example of apparatus employing the principle of this invention, a microwave resonant transmission cavity is connected to a microwave generator and to utilizing equipment so that, when the cavity is tuned to the generator frequency, maximum energy reaches the utilizing equipment. The cavity contains a polished sphere of yttrium-iron garnet placed at a voltage loop point. The sphere is small related to the cavity size, so that when its resonance frequency differs from the applied microwave system frequency the presence of the sphere has no effect on the cavity transmission, but when its resonant frequency is identical with that of the applied microwave frequency and it is set into ferromagnetic resonance, substantially all of the applied microwave energy is reflected and substantially none reaches the load or utilizing equipment. It is necessary to apply a constant unidirectional magnetic field to the sphere in a direction perpendicular tov the direction of the microwave magnetic vector in the cavity, in order to set the sphere into ferromagnetic resonance. The strength, H, of the magnetic field is such as to cause the resonant frequency, to, very nearly to equal the microwave frequency, in accordance with the relation in which 6 is the magnetomechanical ratio. A mechanical system is arranged to apply a variable mechanical force squeezing the yttrium-iron garnet sphere. When the mechanical force is varied at a modulating frequency or in accordance with a modulating frequency function, the device acts as a modulator. When the mechanical force is switched between two values, the device acts as a microwave switch.

In the operation of this device, a normal or bias force on the sphere causes the microwave frequency to lie on the side of the resonance frequency characteristic curve. Change of force changes operation up or down the curve, resulting in a subsequent change in cavity transmission and in modulation or switching of the microwave energy applied to the load.

One purpose of this invention is to provide a microwave modulator employing a component operated near its magnetic electron resonance point and varying this resonance point by applying a modulating mechanical force to the component.

Another purpose of this invention is to provide a microwave switch employing a component biased at or near its resonance. The frequency at which the resonance occurs is changed by the application to the component of a mechanically deforming step force.

A further understanding of this invention may be sesured from the detailed description and drawings, in which:

FIGURE 1 is an oblique view of a form of the invention.

FIGURE 2 is an enlarged cross-section taken on the line 2-2 of FIGURE 1.

FIGURES 3 and 4 are graphs illustrating the operation of the invention.

FIGURE 5 is a view of another form of the invention, being a cross-section of the waveguide transmission resonant cavity.

FIGURE 6 is an enlarged view of the sphere 24 and associated parts of FIGURE 5.

Referring now to FIGURE 1-, a microwave generator 11 having a frequency, for example, of 8800 m.c.p.s. applies its output to a rectangular hollow waveguide 12 of suitable size for the transmission of energy at this frequency. Two iris openings, 13 and 14, define a portion of the waveguide to serve as a resonant cavity. The dimensions of this cavity are such that it is resonant and transmissive at the applied frequency of 8800 m.c.p.s.

Two sapphire rods 16 and 17 are arranged to pass through the narrow sides of the cavity and to meet at or near the cavity center, at a point of high microwave field intensity.

As best shown in FIGURE 2, the sapphire rod 16 passes through the narrow cavity wall 19 into a threaded boss 21, where it meets a brass rod 22 provided with a pointed head 23. A cap 25 applies bias force through rod 22 to rod 16. The sapphire rod 16 contains a cavity in its inner end for the purpose of capturing a garnet sphere 24 held between the end of rod 16 and a plane inner end formed on rod 17.

The garnet sphere 24 is .020 inch in diameter. It is made of the ferrite known as yttrium-iron garnet, having the composition 3Y O -5Fe O The surface of the sphere is highly polished.

Rod 17 passes through the narrow wall 26 of the cavity into the case of a piezoelectric transducer where it is secured to a brass plug 27. The piezoelectric element consists of a ceramic cylindrical tube 28, silver plated on its inner and outer cylindrical surfaces. The inner surface is secured to the plug 27 and the outer surface is secured to the case or frame 18. An electrical connector 29 provides means for applying electrical potential between the inner and outer cylindrical surfaces of the piezoelectric cylinder 28. The piezoelectric transducer is energized, as shown in FIGURE 1, by voice currents derived from a microphone 32 and applied through amplifier 31.

Two coils, 33 and 34, are positioned on an axis 36 perpendicular to the broad faces of the cavity and are connected in series aiding to a direct current generator 37. The output end 38 of the waveguide 12 is connected to a utilization circuit, here illustrated by the microwave demodulator 39, the output of which is connected to a speaker 41.

In the operation of the microwave device of FIGURE 1, microwave energy is applied from the generator 11 to the resonant cavity consisting of the waveguide volume between the two iris openings 13 and 14. This cavity is tuned to the generator frequency, so that nearly all of the energy is transmitted to the demodulator 39. In the absence of the yttrium-iron garnet sphere 24 the cavity resonance curve is as depicted in FIGURE 3, with a bandwidth of about m.c.p.s. The sapphire rods 16 and 17, being nonconductive, do not affect the operation of the cavity appreciably.

When the sphere 24 is present but is not in ferromagnetic resonance, it neither absorbs nor reflects energy and, as its electrical resistance is very high, it has no measurable effect whatever on the transmission by the cavity of the microwave field applied to it from the generator 11.

In order to produce resonance of the electrons in the atoms composing the sphere, it is necessary to apply a constant magnetic field in the direction of the axis 36 of the coils 33 and 34, and also to apply an alternating field in a direction at right angles to the axis 36. The frequency of resonance, w, is given by Equation 1 and is therefore determined by the magnitude of the field, H, applied in the direction of the axis 36. In order to produce ferromagentic resonance, the frequency, f, of the applied alternating field must equal (0. Thus, the field magnitude, H, is adjusted until w equals 1. Perromagnetic resonance then occurs, having a bandwidth, preferably, of less than gauss. At a center frequency of 8800 m.c.p.s. this represents a frequency bandwidth of 42 m.c.p.s. It is relatively easy, however, to secure a bandwidth of the order of 1 gauss, or 2.8 m.c.p.s., with the described yttrium-iron garnet sphere.

With such a field, and with a reasonably small static mechanical pressure of the sapphire rods applying just enough compressive force to hold it in place, the transmission characteristic of the cavity with its contained resonating sphere is as shown in FIGURE 4. The curve 42, including the dashed portion 42', represents the curve of FIGURE 3 to a very much expanded scale. The reentrant portion 43 of the solid line curve is caused by the ferromagnetic resonance of the sphere. With the required steady magnetic field applied, assume iapplied energy having a microwave frequency of 8797 m.c.p.s. The transmission will then be as indicated by the point 44. As the microwave frequency is increased, the crystal lattice of the garnet begins to absorb power and to decrease in its apparent impedance. At the resonance peak of 8800 m.c.p.s., its impedance has decreased until it is much less than the characteristic impedance of the microwave waveguide and of the cavity. The garnet therefore presents a radical mismatch to the applied microwave field, with the result that all of the micro wave energy is reflected and none is transmitted to the output end 38 of the waveguide. Transmission is zero as shown at point 46.

In order to employ this device as a modulator or switch, it is desirable to operate at a point part way down the reentrant peak. This is done by increasing the mechanical pressure on the sphere 24 by any means, as by pressing the rod 16 against the sphere. This may be termed a bias pressure, and results in reducing the resonant frequency w. This experimentally verified change of resonant frequency by pressure is depicted by the dashed curve 47, the impedance presented by the sphere being indicated at the point 48, so that about of the applied microwave energy is transmitted. If now the piezoelectric transducer 18 is energized by talking into the microphone 32, the transducer vibrates against the end of the sapphire rod 17 in accordance with the voice currents. This alternately decreases and increases the pressure on the sphere 24, so that the ferromagnetic resonance frequency may be increased to that illustrated by the characteristic curve 43 or may be decreased to that of the characteristic curve 49, with corresponding changes in energy transmission represented by the points 46 and 51. Thus the microwave energy transmitted through the cavity is amplitude modulated between nearly zero and nearly maximum.

It is obvious that any mode of producing mechanical pressure on the sphere can be employed, and the pressure can be a step function or any other function. The pressure may be applied by means of a mechanical system, a piezoelectric transducer, an electrostatic speaker element or any other transducer or device for converting a signal of any kind into a mechanical pressure function representing the signal.

Although the transducer employed should be one which does not disturb the constant magnetic field applied to the resonant substance, by means of appropriate shielding and other precautions an electromagnetic telephone receiver or speaker moving coil element, or a magnetostrictive transducer can be used.

A toggle system providing mechanical multiplication of the force applied to the garnet sphere is shown in FIGURE 5, with the sphere and its contact elements shown to an enlarged scale in FIGURE 6. In a crosssection through the center of a rectangular waveguide transmission cavity, the waveguide broad faces 52 and 53 are provided with centered apertures for two sapphire rods 54 and 55. The rod 54 protrudes into a boss 21 and cap 25 identical to those shown in FIGURE 2. The rod 54 is provided at its inner end with a concavity 56. a yttrium-iron garnet polished sphere 24, at least .020 inch in diameter when used with microwave frequencies of the order of 8800 m.c.p.s., rests in the concavity. A sapphire rod 58 is provided with a longitudinal gouge 59 having a circular cross-section of radius greater than that of the sphere 24. This rod rests with its gouged slot on the sphere 24. The other end of the rod 58 passes through an aperture in the center of a narrow face 61 of the waveguide and its external end 62 is arranged to have modulating or switching mechanical force applied thereto. This force may be applied by a transducer or by any other means and is applied in the direction of rod length, tending to move the rod lengthwise. In FIG- URE 5 the rod end 62 is terminated in a fiat disc 63. This constitutes a simple transducer for transforming air vibrations or other changes in air pressure against the disc into force on the rod.

The rod 58 is provided with a conical depression 64 directly above the center of the sphere. The pointed lower end of the sapphire rod rests in this depression 64. The upper end of the sapphire rod 55- is also pointed and fits snugly into a conical depression in or near the upper broad internal face 53 of the waveguide. The axis 66 of the sapphire rod 55 is not collinear with the axis 67 of the rod 54 but is slanted away from it by about one degree toward the rod 58. The rod 58 is approximately perpendicular to the axis 67.

The large coils 68 and 69 are connected across a direct current source to provide the magnetic field, H, necessary for operation of the sphere at ferromagnetic resonance,

In the operation of the device of FIGURES 5 and 6, microwave energy is transmitted through the cavity,

' which is tuned to the frequency of the applied energy.

The sphere is set into ferromagnetic resonance at or just off that frequency, as previously explained. Mechanical longitudinal force applied to the rod 58 is multiplied, because of the angularity of the rod 55, relative to the center of the sphere 24, and translated into a force in the direction of the axis 67. This toggle action can be made to secure a mechanical pressure multiplication of about 50.

What is claimed is:

1. A microwave modulator comprising,

a single-crystal sphere of ferrite capable of ferromagnetic resonance,

a microwave resonant cavity,

means applying microwave energy at a selected frequency thereto,

means supporting said sphere at a point of high field intensity in said cavity,

a load connected to said cavity,

means applying a continuous magnetic field to said sphere in a direction perpendicular to the microwave magnetic field plane, the magnitude of said field producing ferromagnetic resonance at a frequency equal to said selected microwave frequency,

means applying an initial biasing mechanical stress to said sphere thereby deforming its crystal lattice and reducing its ferromagnetic resonance frequency and changing the amount of microwave energy transmitted through said cavity,

and means applying a mechanical stress function to said sphere whereby said function appears as an amplitude modulation of the microwave energy transmitted to said load. I

2. A microwave modulator in accordance with claim 1 in which said means applying a mechanical stress function is a transducer.

3. A microwave modulator in accordance with claim 1 in which said means applying a mechanical stress function includes a toggle mechanism for multiplying the force of the applied mechanical function.

4. A microwave modulator comprising,

a microwave resonant cavity adapted to have microwave energy applied thereto,

a first nonconductive rod projecting into said cavity, said rod being provided with a concave depression on the end face thereof within said cavity,

a second nonconductive rod projecting into said cavity and having an end face thereof juxtaposed to said end face of said first rod,

a single-crystal yttrium-iron garnet sphere positioned in the concave depression of said first rod and clamped between the end faces of said first and second rods,

means applying a continuous magnetic field to said sphere in a direction perpendicular to the microwave energy magnetic field plane, the magnitude of said field producing ferromagnetic resonance in said sphere in unstressed condition at a frequency equal to that of the applied microwave energy,

means acting through said rods for applying an initial mechanical stress to said sphere for deforming the crystal lattice thereof and reducing the frequency of ferromagnetic resonance thereof,

and means for applying modulation stress to said sphere through one of said rods to modulate the applied microwave energy.

5. A microwave modulator comprising,

a rectangular waveguide section tuned to resonance as respects microwave energy applied thereto,

a first nonconductive rod projecting through a broad face of said waveguide section into the interior thereof, the end face of said rod positioned interiorly of said waveguide section being provided with a concave depression,

a second nonconductive rod projecting through a narrow face of said waveguide section into the interior thereof perpendicular to said first rod, the interior end of said second rod being provided with a longitudinal groove adjacent to and overlying the concave depression in the end face of said first rod,

a single-crystal yttrium-iron garnet sphere seated in the concave depression in said first rod and clamped between the end face thereof and the grooved end of said second rod,

a pressure lever having a conical end engaging the longitudinal surface of said second rod opposite to said sphere, the longitudinal axis of said pressure lever being inclined at an acute angle to the longitudinal axis of said first rod,

means applying a continuous magnetic field to said sphere in a direction perpendicular to the microwave energy magnetic field plane, the magnitude of said field producing ferromagnetic electron resonance in said sphere in unstressed condition at a frequency equal to that of the applied microwave energy,

means including said first and second rods and said pressure lever for applying an initial mechanical stress to said sphere in a direction along the longitudinal axis at said first rod, said stress being such as to reduce the frequency of electron resonance of said sphere,

and means for applying a modulating mechanical stress to said second rod along the longitudinal axis thereof.

6. A microwave modulator comprising,

a single-crystal element of ferrite capable of ferromagnetic resonance,

a microwave resonant cavity,

means applying microwave energy at a selected frequency thereto,

means supporting said element at a point of high field intensity in said cavity,

a load connected to said cavity,

means applying a continuous magnetic field to said element in a direction perpendicular to the micro- Wave magnetic field plane, the magnitude of said field producing ferromagnetic resonance at a frequency equal to said selected microwave frequency,

means applying an initial biasing mechanical stress to said element thereby deforming its crystal lattice and reducing its ferromagnetic resonance frequency and changing the amount of microwave energy transmitted through said cavity,

and means applying a mechanical stress function to said element whereby said function appears as an amplitude modulation of the microwave energy transmitted to said load,

References Cited by the Examiner UNITED STATES PATENTS 2,898,477 8/1959 Hoesterey 30788.5 2,951,214 8/1960 Bomke et a1. 332-52 X 3,015,787 1/1962 Allin et a1. 333--1.1 3,016,495 1/1962 Tien 3331.1 X 3,087,122 4/1963 Rowen 33194 OTHER REFERENCES Journal of Applied Physics, March 1958. Farrar: Spin Lattice Relaxation Time in Yttrium Iron Garret, pages 425-426, vol. 29, No. 3.

NATHAN KAUFMAN, Acting Primary Examiner.

ALFRED L. BRODY, ROY LAKE, Examiners. 

1. A MICROWAVE MODULATOR COMPRISING, A SIGNAL-CRYSTAL SPHERE OF FERRITE CAPABLE OF FERROMAGNETIC RESONANCE, A MICROWAVE RESONANT CAVITY, MEANS APPLYING MICROWAVE ENERGY AT A SELECTED FREQUENCY THERETO, MEANS SUPPORTING SAID SPHERE AT A POINT OF HIGH FIELD INTENSITY IN SAID CAVITY, A LOAD CONNECTED TO SAID CAVITY, MEANS APPLYING A CONTINUOUS MAGNETIC FIELD TO SAID SPHERE IN A DIRECTION PERPENDICULAR TO THE MICROWAVE MAGNETIC FIELD PLANE, THE MAGNITUDE OF SAID FIELD PRODUCING FERROMAGNETIC RESONACE AT A FREQUENCY EQUAL TO SAID SELECTED MICROWAVE FREQUENCY, MEANS APPLYING AN INITIAL BIASING MECHANINCAL STRESS TO SAID SPHERE THEREBY DEFORMING ITS CRYSTAL LATTICE AND REDUCING ITS FERROMAGNETIC RESONANCE FREQUENCY AND CHANGING THE AMOUNT OF MICROWAVE ENERGY TRANSMITTED THROUGH SAID CAVITY, AND MEANS APPLYING A MECHANICAL STRESS FUNCTION TO SAID SPHERE WHEREBY SAID FUNCTION APPEARS AS AN AMPLITUDE MODULATION OF THE MICROWAVE ENERGY TRANSMITTED TO SAID LOAD. 